“When I’m out at night driving, sometimes you’ll hit pockets of them, and it looks like it’s snowing,” said Janet Bowser, executive director of Wellesley’s Natural Resources Commission,
which spends $20,000 spraying trees on town property each spring to keep them from being devoured by the moths.

“You think they’re just a nuisance, because they’re just around,” said Bowser. “People don’t realize they’re in the process of mating and planting eggs, and they’ll be back.”

But one scientist believes he has found a way to halt the invasion.

“We’re confident that we’re about to fix this problem,” said Joseph Elkinton, a professor of entomology at University of Massachusetts Amherst who has spent the last eight years releasing thousands of flies, known as Cyzenis albicans, in winter moth hot spots.

Their diet is composed exclusively of winter moths, and this year, for the first time, the flies have started to make a dent in the moth population, Elkinton reports. “There’s not too many times in your life you get to fix an ecological problem, but this is one of those times,” he said. “We just have to wait and see. We have more flies than ever before.”

First detected in Massachusetts in Plymouth County and on the North Shore in the late 1990s, the winter moth is an interloper. Native to Europe, the species is here rather mysteriously, and is without natural predators, save for the cold — and even that has been unreliable in recent years.

Last year, winter moths defoliated 89,000 acres of trees across the state, according to the Department of Conservation and Recreation. An early thaw followed by a cold snap killed many this spring,
and their toll went down to about 10,000 acres.

Still, a crude estimate puts their numbers in the trillions, and growing; in the springtime, up to 100,000 caterpillars hatch on a tree and eat the leaves before they bud, leaving lacy skeletons.

“Climate is a limiting factor on this insect. As the winters warm, I expect to see more problems in the interior areas” of the state, said Elkinton.

This year, Elkinton has found winter moths as far inland as Athol, Westminster, and Winchendon. They’re moving west at about 7 kilometers (4.3 miles) per year, he says, and south even faster. Females don’t have wings, so the population spreads tree to tree: the moths drift rather than fly.

The moth’s mating season is November and December. The females lay eggs in trees, and in the spring caterpillars emerge and eat the leaves as they unfurl. Then the caterpillars drop into the soil to pupate until the fall, when the moths emerge and the cycle starts again. Over time, the trees weaken and die.

“The gypsy moth, you have an outbreak that lasts two to three years, there would be a disease agent that causes a collapse. There’s no disease agent like that that we know of for the winter moth,” said Elkinton. “It’s basically a continuous outbreak.”

Some communities have tried to fight the invasion by spraying in the spring. Wellesley uses a nonpesticide spray called Conserve, but $20,000 is only enough to coat about a third of the 3,000 shade trees on town land, and that doesn’t protect trees on private land. Homeowners have flooded landscaping businesses with requests for treatments.

“What really gets the phone ringing is when you get all the caterpillar poop falling out of the trees — you can hear it falling,” said Mike Heffernan, owner of Wellesley-based Lawn Management Corp., who sprays every April and May in communities across the region.
“When they’re eating so much, and pooping so much, it’s falling like rain.”

Spraying works, but it’s not a permanent solution. “You want to have a biological control established,” said Ken Gooch, the state conservation agency’s forest health program director.

Gooch assists Elkinton in the winter moth project, monitoring the defoliation. “You have to look at the big picture, the whole forest of the Northeast, not just here in Massachusetts,” Gooch said. “If you don’t do something about it, you’ll just have thousands and thousands of acres of dead trees.”

Cyzenis albicans could be the answer. Elkinton got the idea from Nova Scotia, where the winter moth appeared in the 1930s. In 1954, the fly was introduced; by 1962, the winter moth was gone.

The flies lay their eggs on the leaves favored by winter moths, and the caterpillars consume the eggs along with the leaves. They hatch inside the pupating caterpillar, which is killed by the fly larvae feeding on it. No moth outbreak in the fall, and,
come springtime, it is the fly that rises from the dirt.

Every year, Elkinton’s team imports the flies from Canada and keeps them cool in a lab on Cape Cod. This year, about 30,000 flies will be released in about 10 spots throughout Massachusetts, Rhode Island, and coastal Maine.

Elkinton said he gets one question a lot: Could the fly itself become a problem?

“The answer is no,” he said. “When the winter moth declines, the fly itself declines. It will basically eliminate its own food supply. The fly and the moth will basically exist at low densities, forever more.”

The winter moth project, however, relies entirely on federal grants to continue; money from the state has dried up, and it’s a battle every year to keep the project alive, said Elkinton.

Gooch had been giving Elkinton a portion of his federal funding, until his budget was cut, too. “If he doesn’t get funded, that work will slow down, and not be done. The flies that are out there will still be out there, but the winter moths will spread fast,” said Gooch.

The winter moth has devastated parts of Rhode Island, Elkinton said, and this year there is an outbreak in Harpswell Neck, Maine. “The winter moth, it has such a large head start. It’s already way ahead,” said Gooch. If Elkinton’s work doesn’t continue, he added, “you might not be able to stop it.”